[en] The long-term safety of geological or shallow disposal systems is evaluated in performance assessment studies. Different scenarios leading to the exposure of man to radioactivity are evaluated, the consequences of the most relevant scenario are analysed, and the estimated dose and risk are compared with appropriate criteria. Accurate hydrogeological modelling is a critical element for the long-term safety assessment of a repository since aquifers are essential components of deep and shallow repository systems for radioactive waste. The Belgian research in this area is focussed on a clay formation (Boom clay). The scientific programme, objectives, achievements of the research in this field, performed at the Belgian Nuclear research Centre SCK-CEN in 1995, are summarized

[en] During the last decade various international initiatives have been taken to investigate the role and feasibility of advanced nuclear energy systems. Such advanced nuclear systems aim at optimizing the utilisation of natural resources and minimizing the generation of long-lived radioactive waste. In the framework of the NEA project Impact of Advanced Nuclear Fuel Cycle Options on Waste Management Policies. SCK-CEN studied the impact of advanced fuel cycles on the geological disposal of high-level radioactive waste in a clay formation.The main objectives of the study are assessing the impact of advanced fuel cycles on the design of repositories for the disposal of high level radioactive waste and on their long-term radiological consequences

[en] During the last 15 years the EC (European Commission) has initiated several research projects on the possibilities of introducing partitioning and transmutation techniques in nuclear fuel cycles in order to reduce the amount of long-lived isotopes present in the radioactive waste. One of these projects, Red-Impact (impact of partitioning, transmutation and waste reduction technologies on the final nuclear waste disposal) started in 2004; its main objective was to assess the impact of partitioning and transmutation on radioactive waste management and geological disposal. The main objective of SCK-CEN's contribution to the Red-Impact project is to evaluate the impact of some representative advanced fuel cycles, making use of partitioning and transmutation techniques, on radioactive waste disposal in a clay formation

[en] The main objectives of SCK-CEN's R and D programme on long-term performance studies are: (1) to develop a methodology and associated tools for assessing the long-term safety of geological disposal of all types of radioactive waste in clay formations and of the shallow-land burial of low-level waste; (2) to assess the performance and to identify the most influential elements of integrated repository systems for the disposal of radioactive waste; (3) to collect geological, piezometric and hydraulic data required for studying the hydrogeological system in north-eastern Belgium; (4) to develop a regional aquifer model for north-easter Belgium and to apply it in the performance assessments for the Mol site; (5) to test, verify and improve computer codes used in the performance assessment calculations of waste disposal concepts and contaminated sites (the computer codes simulate water flow and transport of radionuclides in engineered barriers, aquifers and contaminated sites). The scientific programme and achievements in 1999 are described

[en] Single- or multi-layered surface barriers are used for protecting the underlying hazardous or radioactive waste storage facility and for reducing the risk of dispersion of contaminants to the ground water. To assess the effect of variations in net rainfall rate (total rainfall minus evapotranspiration) on the hydraulic response of a multi-layered barrier, a series of flow simulations were performed. Water fluxes in different layers of the barrier were numerically calculated for three net rainfall rates, i.e., 0.0255, 0.0742, and 0.141 cm/day. These rates correspond to dry, average, and wet climatological conditions in Belgium. Results show that for dry conditions, almost 45% of the water infiltrating the top of the barrier is laterally evacuated. When the net rainfall rate increases from 0.0255 to 0.0742 cm/day, more water is being laterally diverted, and only 22% of the applied rainfall percolates through the barrier. If the rainfall rate is increased from 0.0255 to 0.141 cm/day, the percolation becomes 14% of the applied rainfall. This indicates that fluxes leaving the bottom of the repository increase only slightly, although the rainfall rate for wet conditions is approximately 5 times larger than for dry conditions. Under dry conditions a slightly negative pressure develops in the bottom of the low-permeability clay layer. However, this desaturation does not affect the integrity of the clay

[en] The objective of this report is to assess the performance of disposed medium-level and alpha-bearing waste in a geological repository in the Boom clay formation at the Mol site (Belgium). The results of this study are based on calculations that are in agreement with recent information on the Belgian nuclear programme and the corresponding waste arising. The applied methodology consists of two consecutive steps: (1) a scenario analysis in which relevant scenarios, leading to the exposure of man to radiation are selected, and (2) a consequence analysis in which potential radiological consequences of the exposure are evaluated. The scenario, selected in this study, is designated as the normal evolution scenario and comprises a normal evolution scenario in which the present conditions are assumed to last infinitely. The scenario is extended with a climatic change, a secondary glaciation effects, and a faulting scenario. The applied consequence analysis consists in deterministic and stochastic calculations, which are are complementary. Three pathways of radionuclides to man were considered: (1) the discharge of contaminated groundwater into rivers or (2) into agricultural soils, and (3) the sinking of a water well into the aquifer that overlies the host formation. Calculations indicate that most radionuclides decay within the first metres of the clay barrier. The fission and activation products ¹⁴C, ¹²⁹I, ⁷⁹Se, ⁹⁹Tc, ¹⁰⁷Pd, ⁹³Zr, and ¹³⁵Sr as well as some actinides of the ²³⁷Np and uranium decay series can however reach the biosphere. The maximum dose rates for high-level waste, fuel cladding, medium-level, and iodine waste were calculated. Deterministic calculations indicate that the maximum dose rates are attained via the water well pathway for the case of the climatic change scenario. (A.S.)

[en] In Belgium, the possibility to dispose of high-level radioactive waste in clay formations is studied since 1976. In the PAGIS report, which was the first performance assessment of the disposal of vitrified high-level waste in a clay formation and which was published in 1988, the most important contributors to the total dose via a water well pathway were ²³⁷Np, ¹³⁵Cs and ⁹⁹Tc. Since 1988, several elements that strongly influence the calculated doses have evolved:?the inventory of long-lived mobile fission and activation products in vitrified high-level waste has been improved; the half-life of ⁷⁹Se has been re-estimated; substantial progress has been made in the determination of migration parameters of the main fission and activation products and actinides. In recent performance assessments, the actinides and ¹³⁵Cs do not significantly contribute to the total dose, as they remain confined in the host clay formation during several millions of years due to sorption on clay minerals. Consequently, the total dose resulting from the disposal of vitrified high-level waste or spent fuel is essentially due to releases of mobile fission and activation products. On the basis of recent waste inventory data and parameter values, the most important contributors to the total dose via a water well are: in the case of disposal of spent fuel: ⁷⁹Se, ¹²⁹I, ¹²⁶Sn, ³⁶Cl, and ⁹⁹Tc; in the case of disposal of vitrified HLW: ⁷⁹Se, ¹²⁶Sn, ³⁶Cl, ¹²⁹I, and ⁹⁹Tc. Important remaining uncertainties are the transfer factors of volatile fission and activation products into the vitrified waste during reprocessing and migration parameters of Se. (author)

[en] This contribution to the annual report describes the main activities of the Waste and Disposal Department of the Belgian Nuclear Research Center SCK-CEN. Achievements in 2000 in three topical areas are reported on: performance assessments, waste forms/packages and near- and far field studies. Performance assessment calculations were made for the geological disposal of high-level and long-lived waste in a clay formation. An impact assessment was completed for the radium storage facility at Olen (Belgium). Geological data, pumping rates and various hydraulic parameters were collected in support of the development of a new version of the regional hydrogeological model for the Mol site. Research and Development on waste forms and waste packages included both in situ and laboratory tests. Main emphasis in 2000 was on corrosion studies on vitrified high-level waste, the investigation of localised corrosion of candidate container and overpack materials and the study of the effect of the degradation of cellulose containing waste as well as of bituminized waste on the solubility and the sorption of Pu and Am in geological disposal conditions in clay. With regard to near- and far-field studies, percolation and diffusion experiments to determine migration parameters of key radionuclides were continued. The electromigration technique was used to study the migration of redox sensitive species like uranium. In addition to laboratory experiments, several large-scale migration experiments were performed in the HADES Underground Research Laboratory. In 2000, the TRANCOM Project to study the influence of dissolved organic matter on radionuclide migration as well as the RESEAL project to demonstrate shaft sealing were continued

[en] This contribution to the annual report describes the main activities of the Waste and Disposal Department of the Belgian Nuclear Research Center SCK-CEN. Achievements in 2001 in three topical areas are reported on: performance assessments (PA), waste forms/packages and near- and far field studies. Performance assessment calculations were made for the geological disposal of high-level and long-lived waste in a clay formation. SCK-CEN partcipated in several PA projects supported by the European Commission. In the BENIPA project, the role of bentonite barriers in performance assessments of HLW disposal systems is evaluated. The applicability of various output variables (concentrations, fluxes) as performance and safety indicators is investigated in the SPIN project. The BORIS project investigates the chemical behaviour and the migration of radionuclides at the Borehole injection site at Krasnoyarsk-26 and Tomsk-7. SCK-CEN contributed to an impact assessment of a radium storage facility at Olen (Belgium) and conducted PA for site-specific concepts regarding surface or deep disposal of low-level waste at the nuclear zones in the Mol-Dessel region. As regards R and D on waste forms and packages, SCK continued research on the compatbility of various waste forms (bituminised waste, vitrified waste, spent fuel) with geological disposal in clay. Main emphasis in 2001 was on corrosion studies on vitrified high-level waste, the investigation of localised corrosion of candidate container and overpack materials and the study of the effect of the degradation of cellulose containing waste as well as of bituminized waste on the solubility and the sorption of Pu and Am in geological disposal conditions in clay. With regard to near- and far-field studies, percolation and diffusion experiments to determine migration parameters of key radionuclides were continued. The electromigration technique was used to study the migration of redox sensitive species like uranium. In addition to laboratory experiments, several large-scale migration experiments were performed in the HADES Underground Research Laboratory. In 2001, the TRANCOM Project to study the influence of dissolved organic matter on radionuclide migration as well as the RESEAL project to demonstrate shaft sealing were continued

[en] In view of assessing the validity of the option to dispose of conditioned HLW in an argillaceous formation a site specific approach has been adopted in Belgium. The target formation is the so-called Boom clay formation encountered at the nuclear Mol-Dessel site. The methodology applied in the long-term safety assessments for this option consists of a three steps approach: scenario analysis, consequence analysis and risk assessment. Various techniques (e.g. fault tree analysis and predictive geology) have been adopted for identifying and selecting the relevant assessment scenarios. For the consequence analysis as well stochastic as deterministic techniques have been developed or adapted in order to enable an as comprehensive interpretation of the obtained results as possible. The results of the calculations for the most relevant scenarios, applying the best available site data, indicate dose levels which are far below recommended reference values and a risk level below 1 E-6 per year. The overall safety of the argillaceous repository system is largely dominated by the performances of the host formation itself. 7 refs., 7 figs